the head-disk interface roadmap to an areal density of tbit/in2

Hindawi Publishing CorporationAdvances in TribologyVolume 2013, Article ID 521086, 8 pageshttp://dx.doi.org/10.1155/2013/521086

Review ArticleThe Head-Disk Interface Roadmap to anAreal Density of 4Tbit/in2

Bruno Marchon,1 Thomas Pitchford,2 Yiao-Tee Hsia,3 and Sunita Gangopadhyay2

1 HGST, San Jose, CA 95135, USA2 Seagate Technology, Minneapolis, MN 55435, USA3Western Digital, San Jose, CA 95138, USA

Correspondence should be addressed to Bruno Marchon; [email protected]

Received 10 December 2012; Accepted 12 February 2013

Academic Editor: Tom Karis

Copyright © 2013 Bruno Marchon et al.This is an open access article distributed under theCreative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper reviews the state of the head-disk interface (HDI) technology, and more particularly the head-medium spacing (HMS),for today’s and future hard-disk drives. Current storage areal density on a disk surface is fast approaching the one terabit per squareinch mark, although the compound annual growth rate has reduced considerably from ∼100%/annum in the late 1990s to 20–30%today.This rate is now lower than the historical, Moore’s law equivalent of ∼40%/annum. A necessary enabler to a high areal densityis the HMS, or the distance from the bottom of the read sensor on the flying head to the top of themagnetic medium on the rotatingdisk. This paper describes the various components of the HMS and various scenarios and challenges on how to achieve a goal of4.0–4.5 nm for the 4 Tbit/in2 density point. Special considerations will also be given to the implication of disruptive technologiessuch as sealing the drive in an inert atmosphere and novel recording schemes such as bit patternedmedia and heat assistedmagneticrecording.

1. IntroductionAs the areal density of commercial hard disk drives is quicklyapproaching the terabit per square inch milestone [1–5](Figure 1), the need to improve the reliability of the head-diskinterface (HDI) and to further decrease the head-mediumspacing (HMS) is becoming eversmore critical [3, 6, 7]. LowHMS is a necessary enabler to good writability as well asstrong read-back signal integrity [8, 9]. It is estimated that theHMS will soon need to cross the 7 nmmark in order to reachthis terabit per square inch density point [2, 6]. It is remark-able to realize that the error rate of the stored digital signalthat is being read back improves approximately by about 2xfor every 0.3–0.5 nanometer of decreased HMS. In additionto relentless demand for novel, ultrathin protecting films ofovercoat and lubricant, and subnanometer air gap betweenthe disk and the head, alternative recording technologiespresently being contemplated involve heating the disk to over500∘C (heat-assisted magnetic recording or HAMR) [10–12]and/or physically isolating magnetic bits on small islands ofsub-30 nm in physical dimensions (bit-patterned recordingor BPR) [13–16].

In this paper, the roadmap to an areal density of 4 terabitsper square inches will be discussed. Particular emphasis willbe given to the various spacing components that comprise theHMS budgetand their physical limits. The various implica-tions of recording technologies incorporating HAMR and/orBPR will also be addressed.

2. Historical PerspectiveHard drive technology has constantly evolved to achieveconsistent areal density growth. As one technology such aslongitudinal recording has reached its limit, another suchas perpendicular recording has taken over [18]. Along withadvances in heads, media, signal processing, and servo tech-nology, areal density growth is sustained through improve-ments in the head-disk interface (HDI). Head-media spacing(HMS) is the most important HDI parameter related to arealdensity growth [6]. Continued developments in the tribolog-ical design of disk drives havemaintained the reliability of thehead/disk interface despite decreased spacing.

Analysis of long-term trends shows thatHMShas steadilydeclined over time (Figure 2). The historical trends indicate

2 Advances in Tribology

1990 1995 2000 2005 2010 2015 2020Production year

HDD productsFlash products

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Figure 1: Areal density evolution of HDD and flash memory. After Grochowski [17], with permission.

HM

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Figure 2:Historical variations ofHMS versus areal density from [6].The yellow star is an extrapolation to 4 Tb/in2.

that the HMS of recent products is ∼60% of bit length(Figure 3) [6]. The HMS for recording demonstrations hasbeen typically more aggressive, at ∼50% of bit length.

Research consortia such as the Information Storage Ind-ustryConsortium (INSIC—https://www.insic.com/), StorageResearch Consortium (SRC—http://www.srcjp.gr.jp/), andthe Advanced Storage Technology Consortium (ASTC—http://www.idema.org/?page id=3193) have included collab-orations on HDI technology development. For each arealdensity, the HMS target has typically been set by the Record-ing Subsystem (RSS) groups. The past and current HMS

HM

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m)

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HMS = 0.60∗bit lenght

Figure 3: Historical variations of HMS versus bit length from [6].

trends for the different consortia are shown in Figure 4. TheHMS trends have typically followed the overall trends ofFigures 2 and 3 fairly well. For the HMS roadmaps releasedin 2003–2010, the INSIC trend tended to be lower than forSRC. For the current update (2012), the ASTC trend is higherthan SRC.

For ASTC the main areal density targets are 2 Tb/in2 and4Tb/in2, with scheduled product introductions of 2016 and2020, respectively. The current HMS targets for these twodensity points is ∼5-6 and 4-5 nm, respectively, that is, lessaggressive than those for INSIC. The 4Tb/in2 HMS goal willbe discussed in more details in Section 4.

Advances in Tribology 3

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Figure 4: HMS trends of industry research consortia.

3. Definition of the Head-MediaSpacing Components

As the capacity and performance of disk drives has improved,the mechanical interface has evolved. The spacing betweenthe head and the media has steadily decreased to achieve therapid improvements in areal density. Figure 5 is a diagram ofhead-media spacing (HMS).TheHMS is comprised of the flyheight and coatings (head and disk overcoats, disk lubricant).

In the diagram of Figure 5(b), the surfaces are assumedto be perfectly smooth—they do not include factors such assurface topography and variability of the fly height. The flyheight denotes the spacing between the centerline surfaces ofthe head and disk.The clearance denotes the spacing betweenthe close points of the surfaces. The distinction between flyheight and clearance is depicted in Figure 6.

Below are some definitions of terms commonly used bythe HDI/tribology community.

(1) Head-media spacing (HMS): the spacing betweenthe top of the magnetic layer and the surface of thetransducer.

(2) Flying clearance: difference or margin in fly heightbetween nominal operation and contact between thehead and disk.

(3) Glide avalanche/touchdownheight (TDH): the lowestslider flying height above the mean roughness levelwithout significant slider-disk contact

(4) Flying height = flying clearance + touchdown height

(5) Media lubricant thickness: the average thickness ofthe lubricant, assumed to be on top of the mediaovercoat

(6) Media cvercoat thickness: the average thickness of themedia overcoat, assumed to follow the topography ofthe underlying surface

(7) Head overcoat: head adhesion layer (e.g., silicon) +diamond-like carbon (DLC)

HMS = head overcoat + TDH + clearance

+media lubricant +media overcoat(1)

For current hard drives, HMS reductions have beenachieved mostly through reductions in the clearance anddisk and head overcoats. While this trend will continuefor the immediate future, additional HMS reductions willsoon require notraditional HDI designs in order to meetthe demanding recording requirements for areal densitiesbeyond 1 Tb/in2.

In older drive designs, the air bearing design and passivetopography of the slider determined the fly height and clear-ance of the transducer. In newer drive designs, the fly heightis actively controlled by a signal that changes the shape of theslider. The most common method used for this control is toembed an electrical resistive heater in the slider that will causethe transducer area to protrude closer to the disk, as is shownin Figure 6 [19–21]. At some point the interface may need tobe designed to withstand intermittent or continuous contact[22–27].

Current products have HMS on the order of 10 nm orslightly below [6]. For the 1 Tb/in2 products under devel-opment, a Hi-Lo range bracket can be defined, since thisareal density point is no longer precompetitive. For beyond1 Tb/in2, achievable and stretch values were estimated. Thetechnological development required to achieve the targetswill be discussed in Section 4. The assessment indicatedthat there is promise for achieving 4–6 nm HMS in thelong term. While advancements in HDI design could enablesignificant reduction in HMS, new recording schemes couldadd to the challenge due to new sources of HMS loss:heat assisted magnetic recording (HAMR) will experienceeffects of high temperature at the interface [28–30], and bit-patterned magnetic recording (BPR) could have issues withadded disk topography that would add to the touchdownheight [31–34].

4. ASTC HDI Roadmap to 4Tb/in2 andMajor Research Challenges

As shown in Figure 2, a good estimation of the HMS is∼60% of the length of the bit, and some rationale for this wasrecently proposed [6].This offers a convenient way to projectfuture HMS values, as the following simple equations can bederived. If one defines areal density (AD) in bits per squareinch as the product of the linear density in bit per inch (BPI)and track density (TPI), AD = BPI ⋅TPI. Furthermore, the bitaspect ratio BAR is usually defined as BAR = BPI/TPI; hence,the bit length (BL), in inches, can be expressed as

BL = (AD ⋅ BAR)−1/2. (2)


Slider

ABSAlumina

Transducer

Disk(a)

Transducer orsubstrate

Fly height: ∼5nm

Magnetic media

Total head-media spacing

Overcoat ∼2nm

(HMS) ∼10nm

Lubricant ∼1nmOvercoat ∼3nm

Slider

Disk

(b)

Figure 5: Components of head-disk spacing: (a) schematic of trailing end of head as it flies over disk. (b) Idealized HMS stacks up.

Slider centerline

Slider

Disc centerline Fly height Clearance

Disc surface

Slider surface

Figure 6: Parameters related to head-disk clearance and fly height.

Table 1: Estimated HMS (nm) values as a function of AD and BAR.

BARAD (Tb/in2) 3.0 3.5 4.0 4.52 6.2 5.8 5.4 5.14 4.4 4.1 3.8 3.6

With the HMS approximation of 60% of BL, we now have avery convenient expression linking HMS (in nanometer) toAD (in Tb/in2), for a given BAR:

HMS (nm) ≈ 15 ⋅ (AD ⋅ BAR)−1/2. (3)

Numerical HMS values based on (3) are reproduced in Table 1below.

As shown inTable 1, achieving a 4 Tb/in2 areal densitywillrequire an HMS in the vicinity of ∼4.4 nm, assuming a bitaspect ratio (BAR) of 3. Table 2 offers two scenarios for theHMS breakdown into its various components. In the table,the 2010 projection from INSIC is also presented, as well asa realistic range for the 1 Tb/in2 density point that sponsorcompanies are working towards. For the 4 Tb/in2 point, both“Achievable” and “Stretch” values are offered, correspondingto roughly 60–100%and<60%chance of success, respectively.It is clear from the table that achieving the ∼4.4 nm goalis a high risk, and it will require evolutionary as well asrevolutionary changes in materials, processes, and clearance

control schemes. HDD architecture (e.g., sealing in inertatmosphere) might also be needed [35]. This is discussed inthe following sections.

4.1. Materials: Disk and Head Overcoat. It is clear fromTable 2 that the biggest contributor to today’s HMS isthe carbon overcoat, both on the disk and on the head.Combined, they amount to about half of today’s (aka 1 Tb/in2)HMS budget. Historically, overcoat thickness reduction onboth components has been enabled by denser carbon, as wellas smoother underlying surfaces for the magnetic medium(disk) and RW elements (head). Head carbon has evolvedfrom sputtered to plasma-enhanced chemical vapor depo-sition (PE-CVD) to filtered cathodic arc (f-CAC) carbon[36–39], which increased the density, sp3/sp2 ratio, andhardness [40]. On the disk side, f-CAC technology has notyet been made manufacturable [41, 42], mostly because ofparticle and deposition rate challenges, and most if not alldisks shipped today are coated with some sort of PE-CVDdeposited amorphous carbon overcoat. It is believed thatdisk deposition tools will soon need to offer new carbontechnologies, able to closely emulate a high sp3 bonded,f-CAC-type carbon film. On the head side, evolutionaryoptimization of the overcoat technology might allow to reachthe 4 Tb/in2 goal of ∼1 nm, believed to be the ultimatecoverage limit of f-CAC. Finally, although many attemptshave been made by disk manufacturers to develop and shipnoncarbon overcoated disks, it remains an open questionwhether any thin film materials other than carbon can bemade as hard, dense, and chemically inert as carbon films[43–47]. The latter issue could be alleviated if HDD’s can besealed in an inert environment [35]. It is also believed thatsuch a drastic change in the drive mechanical platform couldhelp reduce overcoat thickness on both heads and disks, asmagnetic medium and read/write alloy corrosion could thenbe suppressed, or at least drastically reduced. Our estimateof such benefit is in the range of 0.2–0.3 nm or greater, foran overall reduction of ∼0.5 nm, that is, a substantial amountof about 10% of the total HMS. Another potential alternativeis to develop a disk surface modification process such thatthe highly corrosion-susceptible media material surface is


Table 2: HMS breakdown scenarios for 4 Tb/in2.

HMS Budget 1 Tb/in2 4Tb/in2 HMS adderComponents INSIC 2010 Target HMS by ∼2014 INSIC 2010 Target HMS by ∼2020-21 (effect of HAMR, BPM)Component Hi Lo Medium risk High risk BPM HAMRTDH 1.8 2.0 1.0 1.4 1.1 0.6 0.3 0.5Disc overcoat 0.9 2.5 2.0 0.6 1.8 1.5 0.3 0.6Disk lubricant 0.9 1.2 1.0 0.8 1.0 0.8Clearance 1.3 1.2 1.0 0.6 0.6 0.5 0.2 0.3Head overcoat 1.0 2.0 1.5 0.7 1.1 0.9Total 5.8 8.9 6.5 4.0 5.6 4.3 0.8 1.4The HMS adder columns corresponding to HAMR and BPM will be discussed in Section 4.

treated or modified without contributing to spacing loss witha creation of a nonfunctional or weakened layer of magneticmaterial on the media surface [48, 49].

4.2. Materials: Disk Lubricant. Table 2 shows that a 0.2 nmreduction of lubricant thickness from 1.0–1.2 to 0.8–1.0 isneeded. This does not seem like much, but with today’sextremely tight reliability/HMS margins, this change isactually significant, and it is believed that inventions willbe needed to reach those goals [50, 51]. The lubricantindustry is now more diversified than before, and newlubricant structures are now routinely offered by at leastthree different companies. It remains to be seen whetherconventional lubricant chemistries (functionalized perflu-oropolyether) will be able to achieve this thickness goal.Perhaps unconventional approaches, such as direct surfacetreatment/functionalization of the disk overcoat will beneeded [48].

4.3. TDH: Topography. Touchdown height (TDH) globallydefines all residual disk and head topographies that preventthe head from coming into close proximity to the magneticmedium (Figure 6). It is affected by waviness (∼1–1000𝜇mwavelength range) and roughness (<1𝜇m) of the substrate[52–54], as well as the nanoroughness of the magneticfilm-overcoat structure of the disk [55]. Unlike materialsthicknesses (lubricant, overcoat), TDH is not a requirementfor proper HDD reliability and could, in theory, be broughtto zero. It assumed that engineering evolutionary optimiza-tion of polishing (disk substrate, slider) and deposition(media/overcoat) processes will be possible to achieve theHMS goal. Finally, it has been proposed that disk lubricant“roughness”, both at the nanoscale (conformation) [56] andthe microscale (thickness modulation or “moguls” [57] and“ripples” [58, 59]), also contributes to TDH, and lubricantoptimization, as discussed previously, will be needed to alsolower its contribution.

4.4. Head-Disk Clearance. Of all the HMS contributors,clearance has probably exhibited the largest decrease in thelast 10 years or so, thanks to the advent of thermal flyingheight control (TFC-Figure 7) [19, 21]. This revolutionaryapproach has allowed the HDD industry to achieve a 10-foldreduction in clearance fromca. 10 nm ten years ago to∼1.5 nm

Slider(not to scale)

Disk

Resistive heaterRW element

∼1-2nm

Figure 7: Schematics of the active fly height control scheme usingan embedded resistive heater.

today (Table 2). To achieve the 0.6 nm ASTC clearance goaland to easily compensate for clearance changes inducedby temperature, altitude, or humidity, further enhancementof clearance control will be needed such as closed-loopTFC control. “Surfing” of light contact recording has alsobeen suggested [26], but it remains to be seen whether thisapproach can be made reliable.

5. Tribology and HDI Challenges forAlternate Technologies

As discussed earlier, to advance HDD magnetic recordingbeyond the ∼1 Tb/in2 believed achievable with PMR/SMR,HAMR and BPM technologies are being developed. It isbelieved that HAMR will first be introduced. To go beyond∼6 Tb/in2, the ASTC roadmap shows that HAMR will beaugmented with BPM recording technology [11]. While thesewill enable significant gains in areal density, they will presentchallenges for the HDI.

In terms of HMS, the effect of these technologies isshown in the “HMS Adder” columns of Table 2. The touch-down height is increased due to added media roughnessfor HAMR or residual topography for BPM [32, 60]. Theincreased roughness also drives the need for added mediaovercoat thickness to maintain adequate coverage. Withregards to clearance, HAMR would require added margin asa guardband for possible thermal protrusion of the near-fieldtransducer (NFT) [10], whereas BPM will require clearance


margin as a guardband against residual topography that couldinduce head wear. It is believed that BPM will not requirehead overcoat thickness adder.

As the HDD industry moves towards the introduction ofHAMR recording technologies, there is a high demand forthe HDI to demonstrate robustness at higher temperatures[30, 61–63]. If the current disk overcoat and lubricant films arenot capable of withstanding the high recording temperature(>500∘C), newmaterials will be needed.On the head side, it iscurrently believed that the temperature should be limited to∼150∘C in order to provide long-term interface reliability to thecurrent overcoat material. In any event, further developmentin tribologicalmaterials for both heads andmedia seem likelyneeded in order to insure proper reliability of the HAMRinterface.

The later introduction of BPM recording technologywill present further challenges for HDI design. Residualtopography left behind by the BPM manufacturing processchallenges air bearing designers to design a slider that canbetter follow residual topography [34, 64, 65] as well asmedia manufacturers to have a uniform overcoat and lubri-cant coverage of the media surface. Moreover, the residualmedia topography could lead to excessive induced lubricantroughness on the microscale (lubricant moguls and ripples)unless a new lubricant can be designed.

6. ConclusionIn summary, head-media spacing continues to be an enablerfor continued march to increase the areal density to fendoff the assault and encroachment by nonvolatile solid-statememory technologies. To maintain HDD leadership andcompetitive edge in the data storage arena, the industry mustcontinue to develop new component technologies to supportthis areal density advance. However, to be successful, thedesign of the head-disk interface must be an integrated effortwhere the component technologies are developed in concertto solve the system problem. The success of the areal densitygrowth will depend on how successful we, as an industry, arein integrating this development.

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